Polar drift has been discussed in [1]} and [2]} and the free Chandler wobble in [3]}. In the present paper, we focus on the third major component, the forced annual oscillation. This oscillation is often assumed to be forced by the Earth's fluid envelopes, hence also by climate variations (e.g. [4]}, [5]} chapter 7, [6]}, [7]}). The fact that the observed Chandler period (433-434 days) is not equal to the theoretical value of the Euler period (306 days) has been interpreted in terms of Earth elasticity. The annual period is forced by interactions with fluids. Variations in the Sun-Earth distance, hence of the corresponding gravitational forces, displace the fluid atmosphere and ocean; exchanges in angular moment affect RP and lod. The annual oscillation of the rotation pole RP (coordinates \(m_1\)  and \(m_2\) ) would therefore be due to the presence of the fluid envelopes. If Earth were devoid of fluid envelopes, the annual oscillation should not exist, which would contradict both the theory and observations of motion of a Lagrange top [8]}. [9]} recall that [10]}, [11]} “demonstrated that the annual wobble was forced, at least in part, by the seasonal migration of air masses on and off the Asian continent”. [12]} showed that the annual fluctuation in water storage on the continents was also important. [13]} re-examined the sources of annual wobble excitation and concluded that the air mass effect accounted for much but not all of the annual wobble, and water storage did not explain the remainder. Reservations appear in chapter 7 of [5]}, Seasonal Variations, page 146, who writes: “The principal seasonal oscillation in the wobble is the annual term which has generally been attributed to a geographical redistribution of mass associated with meteorological causes. Jeffreys, in 1916, first attempted a detailed quantitative evaluation of this excitation function by considering the contributions from atmospheric and oceanic motion, of precipitation, of vegetation and of polar ice. Jeffreys concluded that these factors explain the observed annual polar motion, a conclusion still valid today, although the quantitative comparisons between the observed and computed annual components of the pole path are still not satisfactory”. In summary, the interpretation of the forced annual component of polar motion is still hypothetical and fails to be validated by a numerical model.
